all of the attempts are based on the idea of tor ##qu ##ose ##le ##ct ##ivity ; selecting one direction for the vinyl groups to " rotate " in turn sets the stereo ##chemist ##ry as shown below . silicon @ - @ directed na ##zar ##ov cy ##cl ##izations can exhibit induced dia ##ster ##eo ##sel ##ect ##ivity in this way . in the example below , the si ##ly ##l @ - @ group acts to direct the cy ##cl ##ization by preventing the distant al ##ken ##e from rotating " towards " it via un ##fa ##vor ##able ste ##ric interaction . in this way the silicon acts as a trace ##less auxiliary . ( the starting material is not en ##ant ##io ##pur ##e
all of the attempts are based on the idea of torquoselectivity ; selecting one direction for the vinyl groups to " rotate " in turn sets the stereochemistry as shown below. silicon @ - @ directed nazarov cyclizations can exhibit induced diastereoselectivity in this way. in the example below, the silyl @ - @ group acts to direct the cyclization by preventing the distant alkene from rotating " towards " it via unfavorable steric interaction. in this way the silicon acts as a traceless auxiliary. ( the starting material is not enantiopure